It is known to mold products from a pulp slurry by dipping a porous mold into a pulp slurry and subsequently drying and optionally pressing the thus molded product. Examples of such products are egg cartons, shock absorbing packaging inserts and paper trays, paper cups, drink carry out trays, mushroom and berry boxes and other forms of industrial, agricultural and consumer packaging.
Porous pulp molding dies have been made of a woven wire cloth material, which is stretched to conform to a die surface. Such dies have some disadvantages in terms of the amount of distortion or stretching of which the wire cloth is capable to enable it to conform to the die surface. Further disadvantages include the propensity of the wire cloth to rupture. The use of wire cloth is also associated with some limitations on the complexity of the products that can be molded. In particular, when forming a wire cloth into a mold, the pores of the wire cloth will be deformed, and so it is not possible to control the distribution of the openings.
Yet another disadvantage is the cost of making such molds: as the wire cloth is typically not self supporting, it will be necessary to provide also a metal backing which is specific for the product that is to be molded. The tools are moreover prone to clogging and difficult to repair.
It is also known, from e.g. U.S. Pat. No. 3,067,470, to provide a porous pulp molding die from small spherical bodies, which are sintered together so as to provide a porous body. The bodies may be made from polymer material as disclosed in U.S. Pat. No. 3,067,470. However, dies of this type suffer not only from disadvantages in terms of strength and limited temperature range in which they may be used. They also suffer from a trade-off between surface quality and pressure drop: the finer the particles used at the surface, the smaller the channels will be and thus the greater the pressure drop.
WO2011059391A1 discloses a method of making a pulp molding die by sintering together particles of a metallic material, such as bronze. While such a die may withstand higher temperatures as compared to the polymer based die, its manufacturing is associated with a more difficult sintering process, as higher temperatures are required. Moreover, the finished die suffers from the same advantages as that made of polymer material.
Hence, several challenges remain with respect to the molding of products from pulp: It would be desirable to provide smoother surface structures, to reduce energy consumption, to provide a less expensive process for making the mold and to provide a mold that is durable and can be subjected to elevated temperatures. There is also a desire to provide improved quality control of the forming process.